This invention relates to control means for an aircraft propulsor of the class known as Q-Fans.sup.TM being developed by the Hamilton Standard Division of United Aircraft Corporation and particularly to coordinated means for controlling the pitch of the fan blades, engine fuel flow and fan exhaust nozzle area for reversing the fan blade angle through feather.
To more fully understand the Q-Fan.sup.TM, reference should be made to U.S Pat. No. 3,747,343 granted to Mr. George Rosen and assigned to the same assignee. As is the case with all controls for gas turbine power plants, it is customary to provide means for monitoring engine operations and provide control means to convert those signals to a logic that will provide, as best and efficient as possible, optimum engine operations. Thus the control manifests these signals to provide fast thrust response during take-off and landing, optimum TSFC (Thrust Specific Fuel Consumption) in all cruise conditions, while preventing stall or surge, rich or lean blowout, overtemperature, overpressure and overspeed conditions.
Obviously, the incorporation of such variables as variable pitch fans, variable area exit nozzles and the like will add complexity to the control system. Application Serial No. 477,532 filed on the same data by Roy W. Schneider and Kermit I. Harner, entitled "Control System for Variable Pitch Fan Propulsor" and assigned to the same assignee described a reliable coordinated control that coordinates fuel flow to the gas turbine engine and pitch change of the fan blades and the area of the variable exit nozzle of the bypass duct so as to achieve rapid thrust modulation in takeoff and landing modes and optimum TSFC in all cruise and long duration flight conditions while providing the typical protection to the gas generator. In particular this aforementioned application discloses control means biasing the power lever schedule with flight Mach No. to provide control of engine fuel flow, fan pitch and area of the exit nozzle in the event this variable is included. The surge of the fan is prevented by defining a scheduled exhaust nozzle area which is a function of flight Mach No. and corrected engine fan speed (N.sub.F / .sqroot..theta.) and feeding it to a selector circuit that selects the larger of the normal scheduled area and the minimum fan exit area which is required to avoid surge. The fan exit area nozzle is also utilized to optimize performance (TSFC) for long duration flight conditions. Except for the condition lever which is typically employed in aircraft for starting, shutting-off and feather, a single power lever is so coordinated to provide engine fuel flow, variable fan pitch change and variable area exhaust nozzle control throughout the operating range.
This invention is particularly directed to means to reverse the pitch of the fan through feather as opposed to passing through flat or zero pitch. This presents a significant problem since the pitch of the fan just prior to reversing is at a low positive blade angle and must move to a higher positive blade angle to reach the feather angle position. Without anything else being done the higher positive pitch will increase the blade loading and produce a higher positive thrust which is obviously undesirable inasmuch as this increases forward flight velocity where a decrease is required. Of course, once in reverse pitch maximum reverse thrust is obtained. To achieve this end we have found means to coordinate the coordinated functions of engine fuel flow, blade angle and exhaust nozzle area so as to minimize forward thrust by judiciously reducing and increasing fuel flow and/or increasing exit fan nozzle area and optimize the transient response.
Thus, in summary, without limiting the scope of this invention the salient features are:
1. The ability to provide rapid thrust response in the takeoff and landing conditions by coordinating fan pitch and engine fuel flow so as to optimize transient response characteristics.
2. The ability to go from forward thrust to reverse thrust (through feather) rapidly while maintaining satisfactory shaft torque conditions and minimizing increase thrust excursion by coordinating fan pitch, exhaust nozzle area and engine fuel flow so as to optimize transient response characteristics. For example, the time calculated from digital simulation to obtain substantially 100% reverse thrust from 100% forward thrust was approximately between 1.3 to 2.0 seconds. This compares with a typical engine reverse thruster that requires substantially 8 to 10 seconds.
3. The ability to modulate thrust smoothly from maximum to near zero thrust in both forward and reverse range on the ground.